This invention relates to dynamoelectric machines and more particularly to an internally mounted fan for a fractional horsepower electric motor.
Typically, a fractional horsepower electric motor includes a stator assembly having a stack of laminations (preferably made of sheets of suitable steel or other ferro-magnetic material) punched to have a central bore and a plurality of winding receiving slots extending generally radially outwardly from the bore. The laminations are stacked to form a core and are securely fastened together. A rotor assembly including a rotor, typically of squirrel cage design, and an axial shaft extending from both ends of the rotor is mounted in the stator. Specifically, the rotor is received within the bore of the stator core and centered therewithin. A substantially uniform air gap is disposed between the rotor and the portions of the stator core forming the bore. These motors typically include a pair of end shields secured to the ends of the stator assembly. The end shields each include a bearing in which one end of the rotor shaft is journalled so that the bearings rotatably support the motor within the stator assembly. Such motors are frequently enclosed in a motor shell which includes a motor frame and the two end shields.
In an enclosed motor of this type, it is desirable to provide some means for cooling the stator coils. These coils during operation have current flowing through them to generate a rotating magnetic field and this current generates heat in the stator coils. Excessive heat is of course undesirable and could lead to failure of the motor. As a result, various fans have been proposed in the past to cool the exposed portions of the stator coils. Such fans have generally been of cast metal construction, and as a result have had to be spaced a substantial distance from the stator coils themselves in order to provide sufficient electrical clearance. Such fans typically have had a generally closed metal back supporting a series of vanes. It is believed that the vanes in these previous designs provided substantially all of the air movement function of the fans.
Although these previous fans have worked well for their intended purpose, they could be improved. For example, some fractional horsepower electric motors have stator coils which extend past the rotor and as a result have both exposed end turns and exposed side portions of the stator coils which should be cooled. However, with many present fans, the air next to the exposed inner portion of the stator coil and in some cases the air next to the end turns themselves is relatively still or stagnant. That is, some present fans cause air to flow around the outside of the stator coils but are believed to allow air to remain stationary along the end turns and along the exposed inner portion of the stator coils. This results in generally adequate cooling of the outside of the stator coils but less than desirable cooling of the exposed inside portions and end turns of the stator coils. Since the current is the same in the outer portion of the stator coil and in the inner portion, one would want approximately the same air flow over both portions of the coil. Prior fans have also generally been constructed of metal. This provides a reliable fan but it does require that the fan be placed at least a certain distance from the coils in order to receive the approval of various agencies such as Underwriter's Laboratories. This additional spacing, however, results in reduced air flow at the coils themselves, which is less than desirable.